This invention relates to a magnetron, especially to getters for absorbing residual gas prevailing inside the magnetron.
Generally, as a magnetron generates efficiently microwaves, it may especially be incorporated in a microwave oven, and widely used for defreezing or heating foods. Accordingly, improvement of its life, safeness and quality has eagerly been requested.
FIG. 1 is a fragmentary, sectional view showing one example of conventional magnetron generally used. In this figure, numeral 1 designates a cylindrical anode made of oxygen free copper, for example, and 1a a plurality of vanes secured, in equally spaced angular relationship, to the inner wall of the anode cylinder in radial direction with respect to a cathode filament 2.
The anode cylinder 1 and vanes 1a constitute an anode structure. The cathode filament is made of a helical thorium-tungsten wire having a carbonized surface, for example. The helical cathode is arranged concentric with the axis of the anode cylinder 1 so that a so-called interaction space is formed between the cathode filament and the vanes 1a. Flange-shaped upper and lower end shields 3 and 4 each made of such a high melting metal as molybdenum or tungsten are secured to both end portions of the cathode filament 2 by welding, for example, and disc shaped getters 5 and 6 made of such a metal as zirconium are secured by welding to opposing surfaces of the flanges of upper and lower end shields 3 and 4. The upper end shield 3 is supported by central support 7 and the lower end shield 4 by side supports 8. The central support 7 and side support 8 also act as electrical lead wires and supply electric current to the cathode filament 2 from an external power supply (not shown). The frust-conical magnetic pole pieces 9 and 10 made of iron, for example, which are secured to both ends of the anode cylinder 1 direct the magnetic flux from an external permanent magnet 12 as shown at two dot chain line in FIG. 1 into the interaction space. When a current is supplied to the cathode filament 2 through the central support 7 and side support 8, thermionic electrons given off by the cathode filament 2 are discharged into the interaction space to induce an oscillation. With this construction of the conventional magnetron, however, the disc shaped getters 5 and 6 made of zirconium, for example, are secured by welding to respective opposing surfaces of the upper and lower end shields 3 and 4, that is, the surfaces of getters 5 and 6 are arranged to face the interaction space. Therefore, if the dimension and surface area of the getters 5 and 6 are increased or the purpose of greatly promoting the absorption effect for the residual gas in the magnetron, the output high frequency electromagnetic field would be disturbed so that it is difficult to provide large size getters for the conventional magnetron. Furthermore, since the upper and lower end shields are discs each having an outer diameter of about 8 mm and the central support 7 having a diameter of about 5 mm extends through the center of the end shields, the effective area roomed for the getters 5 and 6 is relatively small. Further, the end shield for supporting the getter is made of high melting point metal. For these reasons, securing and welding of getters are difficult and mounting of getters on the end shields is usually carried out by spot welding. However, since the cathode filament 2 made of a thorium-tungsten carbide having a small mechanical strength is secured to the opposing surfaces of the upper and lower end shields 3 and 4, it is difficult to weld the filament without damaging the cathode filament. Furthermore, the welded getters undergo thermal deformation due to the heat generated at the time of operation, and sometimes a spark strikes across a small gap (of about 0.6 mm) between the free end portions of vanes 1a and the getters thereby causing insulation breakdown. Although getters 5 and 6 may possibly be mounted at other portions than the interaction space which tends to be adversely affected by the mounting of the getters, most of the portions inside the magnetron participate in propagation in high frequency energies and thus, locations suitable for supporting the getters are limited to avoid undesirable disturbance of the electromagnetic field and degradation of the output characteristics. Furthermore, as the material of anode comprises copper which has low weldability, great task remains for selecting positions suitable for supporting the getter at which getter activation temperature is optimum and undesirable output characteristics are not caused.